1. Field of the Invention
This invention relates to methods of maintaining the wear resistance of silicon nitride, alumina, zirconia and other wear-resistant ceramic structural materials even in the presence of water, and can be used in the lubrication of sliding areas in environments in which water is present, or in the lubrication of sliding areas which are required to be fire-resistant due to disaster prevention considerations in coal mines, hydraulic equipment and the like.
2. Prior Art Statement
Silicon nitride, alumina, zirconia and other engineering ceramics have mechanical properties which are greatly superior to metals in the areas of heat and wear resistance, and thus are expected to have applications in cutting tools, bearings, high-efficiency engines, high-efficiency gas turbines and the like.
Silicon nitride appears particularly promising because it is composed of elements which are plentifully available anywhere, in contrast to conventional cemented carbides composed of rare and unevenly distributed elements such as tungsten, cobalt, nickel and such.
When metal is used to make machine parts to be operated in environments in which water is present, the problem of rusting occurs, so ceramic materials which have no danger of rusting are suitable for these kinds of applications.
In addition, lubricating oil, solid lubricants or other lubricants which carry with them the danger of fire cannot be used to lubricate sliding parts in coal mines or hydraulic equipment which are required to be fire-resistant from disaster prevention considerations. Thus water-based lubricants are useful for these applications, but metals would not suitable due to the problem of oxidation, so ceramics are expected to be used in these kinds of applications.
Yet when these ceramic materials are subjected to friction, particularly at low speeds, both their coefficient of friction and wear losses become large and unstable (H. Tomizawa and T. E. Fischer, ASLE Trans. 30 (1987) 41.). This trend is particularly pronounced in silicon nitride, in which wear is severe even when a small amount of water contaminates another organic solvent.
The reason for this is that when silicon nitride is subjected to friction under water, the surface of the silicon nitride is oxidized into silicon dioxide which reacts with water to form a hydroxide. In other words, hydrogen, a component of water, is bonded onto the surface of ceramics to form silanol groups (.fwdarw.Si--OH) whose silicon atoms produce tetrahydroxysilane (Si(OH).sub.4) soluble in water by nucleophilic substitution reaction of water with the silicon atoms. Since the tetrahydroxysilane is eluted into the water, tribochemical wear takes place.
Since alumina, zirconia and other oxide ceramics are composed of oxides to begin with, they similarly react with water which causes corrosion cracking, resulting in friction and wear becoming unstable (S. Sasaki, Proc. Inter. Conf. Wear of Material, Denver (1989)).
Thus as described above, these ceramics are not suitable as structural materials for sliding parts in environments in the presence of water and thus improvement of the water lubricity of these ceramics is strongly desired.